Crack-stimulated generation of deformation twins in nanocrystalline metals and ceramics

A theoretical model is proposed that describes the generation of deformation twins near brittle cracks of mixed I and II modes in nanocrystalline metals and ceramics. In the framework of the model, a deformation twin nucleates through stress-driven emission of twinning dislocations from a grain boundary distant from the crack tip. The emission is driven by both the external stress concentrated by the pre-existent crack and the stress field of a neighbouring extrinsic grain boundary dislocation. The ranges of the key parameters, the external shear stress, τ, and the crack length, L, are calculated within which the deformation-twin formation near pre-existent cracks is energetically favourable in a typical nanocrystalline metal (Al) and ceramic (3C-SiC). The results of the proposed model account for experimental data on observation of deformation twins in nanocrystalline materials reported in the literature. The deformation-twin formation is treated as a toughening mechanism effectively operating in nanocrystalline metals and ceramics.     

Investigation of activities of grain boundaries in nanocrystalline Al under an indenter by a multiscale method

Activities of grain boundaries in nanocrystalline Al under an indenter are studied by a multiscale method. It is found that grain boundaries and twin boundaries can be transformed into each other by emitting and absorbing dislocations. The transition processes might result in grain coarsening and refinement events. Dislocation reflection generated by a piece of stable grain boundary is also observed, because of the complex local atomic structure within the nanocrystalline Al. This implies that nanocrystalline metals might improve their internal structural stability with the help of some special local grain boundaries.     

Investigation of grain boundary activity in nanocrystalline Al under an indenter by using a multiscale method

Grain boundary activity in nanocrystalline Al under an indenter is studied by using a multiscale method.It is found that grain boundaries and twin boundaries can be transformed into each other by emitting and absorbing dislocations.The transition processes might result in grain coarsening and refinement events.Dislocation reflection generated by a piece of stable grain boundary is also observed,because of the complex local atomic structure within the nanocrystalline Al.This implies that nanocrystalline metals might improve their internal structural stability with the help of some special local grain boundaries.     

Simulated defect growth avalanches during deformation of nanocrystalline copper

In this work we introduce a method to capture the proliferation of material defects that carry inelastic deformation, in microstructures simulated through isobaric–isothermal molecular dynamics. Based on the premise that inelastic dissipation is accompanied by a local temperature rise, our method involves analyzing the response of a chain of Nosé–Hoover thermostats that are coupled to the atomic velocities, while the microstructure deforms under the influence of a ramped external stress. We report results obtained from the uniaxial deformation of two nanocrystalline copper microstructures and show that our analysis allows the dissipative signal of a variety of inelastic events to be effectively unified via an ‘avalanche’ of dissipation. Based on this avalanche, we quantitatively compare dissipation for inelastic deformation under tension vs. compression, observing a significant tension–compression asymmetry in this regard. It is concluded that the present method is useful for discerning critical points that correspond to collective yield and inelastic flow.     

A new micromechanics-based scale transition model for the strain-rate sensitive behavior of nanocrystalline materials

An original two-step “three phase” elastic–viscoplastic scale transition model is developed based on the combined self-consistent and Mori–Tanaka schemes. A coated inclusion is embedded within a matrix, wherein the inclusion represents grain interiors and the coating of the inclusion mimics the effects of grain boundaries and triple junctions. The predominant behavior within the grain interiors is captured through dislocation glide, whereas grain boundary (GB) dislocation emission and absorption, as well as thermally assisted GB sliding, describe the deformation processes within the coating describing the GB affected zone. Furthermore, an imperfect interface is assumed between the inclusion and the coating to account for viscoplastic grain boundary sliding along a stick-slip mechanism. Results and discussion focus on the competitive roles of GB sliding, GB dislocation emission/absorption, dislocation sweep in grain cores and collective dislocation plasticity, and the origins of the pronounced strain rate sensitivity of fcc NC materials.     

Effect of cooperative nanograin boundary sliding and migration on dislocation emission from a blunt nanocrack tip in nanocrystalline materials

Theoretical model is suggested that describes the effects of the cooperative nanograin boundary sliding and stress-driven nanograin boundary migration (CNGBSM) process on the lattice dislocation emission from an elliptically blunt nanocrack tip in deformed nanocrystalline materials. Within the model, CNGBSM deformation near the tip of growing nanocrack carries plastic flow, produces two dipoles of disclination defects and creates high local stresses in nanocrystalline materials. By using the complex variable method, the complex form expression of dislocation force is derived, and critical stress intensity factors for the first lattice dislocation emission are obtained under mode I and mode II loading conditions, respectively. The combined effects of the geometric features and strengths of CNGBSM deformation, nanocrack blunting and length on critical SIFs for dislocation emission depend upon nanograin size and material parameters in a typical situation where nanocrack blunting and growth processes are controlled by dislocation emission from nanocrack tips. It is theoretically shown that the cooperative CNGBSM deformation and nanocrack blunting have great influence on dislocation emission from blunt nanocrack tip.     

Growth dynamics and thermal stability of Ni nanocrystalline nanowires

The growth dynamics and the thermal stability of a Ni nanocrystalline nanowire (NNW) model system fabricated using electrochemical deposition has been investigated using X-ray diffraction, scanning and transition electron microcopy, and differential scanning calorimetry (DSC). It has been found that the thermal stability of the Ni-NNW is dominated by the microstructure movement and the grain boundary rotation mechanism at temperature ranging from 400 to 600 °C. The Ni-NNW experiences the Rayleigh instability at temperature approaching the melting point. The observed fragment separation in the Rayleigh phase-transition is much greater than that expected theoretically.     

Atomic Structure and Properties of the (310) Symmetrical Tilt Grain Boundary (STGB) in SrTiO3. Part I: Atomistic Simulations

The relaxed structure and energy of the (310) symmetrical tilt grain boundary (STGB) in SrTiO₃ have been calculated using static lattice energy minimization methods. In principle, the (310) GB plane can either be a cation-rich, positively charged SrTiO plane or a negatively charged oxygen plane, and both scenarios have been considered in this report. The effect of point-defect reconstruction at the GB core region, manifested either as completely missing columns or as half-filled columns of ions as suggested by experiments, has been analyzed. The results indicate that while Schottky defects are very strongly preferred energetically at the GB core, there is not significant gain in energy by having half-filled columns, as opposed to fully-dense and fully-empty columns, at the GB core. The simulation results have been analyzed in the context of Pauling's rules of crystal chemistry and bicrystallography. The results form the basis for an objective comparison with experimental studies in Part II of the paper.     

Annealing behaviour of nanocrystalline NiTi (50 at% Ni) alloy produced by high-pressure torsion

An equiatomic nanocrystalline NiTi alloy, deformed by high-pressure torsion (HPT), was investigated. The as-prepared bulk NiTi alloy consisted of both amorphous and nanocrystalline phases. Crystallization and structural changes during annealing were investigated by differential scanning calorimetry (DSC), X-ray diffraction (XRD) and transmission electron microscopy (TEM). DSC thermograms and X-ray analyses revealed stress relaxation and partial crystallization below 500?K, while grain growth of the nanocrystals occurred predominantly after heating to temperatures above 573?K. Along with the amorphous phase crystallization, a continuous growth of pre-existing nanocrystals that are retained after HPT was observed. The DSC signals observed during continuous heating experiments indicate an unusually large separation between the crystallization and growth stages. A detailed analysis of the evolution of the enthalpy release upon annealing revealed reproducibly non-monotonous trends with annealing temperature that cannot be explained solely by nucleation and growth of crystalline volume fractions. Instead, the results can be rationalized by assuming a reverse amorphization process occuring during annealing at 523?K. This behavior, which also caused a large variation in nanocrystal size after annealing at higher temperatures, is discussed with respect to the nanoscale microstructural heterogeneity after initial deformation processing.     

Shear band formation and fracture behavior of nanocrystalline (Co,Fe)-based alloys

The crystal structure, crystallization, fracture behavior and mechanical properties of (Co_{1 ? x} Fe _x )₈₉Zr₇B₄ (x = 0–0.7) nanocrystalline ribbons were investigated. The crystallization peaks of the amorphous ribbons tend to shift to higher temperatures with increasing Fe content. After annealing at 475°C for 3600 s, the main crystallization product is hcp-(Co,Fe) for the Co-rich composition (x = 0), bcc-(Co,Fe) for high Fe contents (x ≥ 0.3) and a mix of bcc, and fcc for intermediate compositions (0.025 ≤ x ≤ 0.15). The relative strain at fracture decreases dramatically (ε_f < 0.01) for x ≥ 0.15, whereas for lower Fe content it has a maximum (ε_f > 0.037) at x = 0.025 and 0.050 resulting in excellent resistance against fracture. The brittle ribbons (x ≥ 0.15) showed smooth fracture surface with dimples less than 230 nm in diameter, small localized or absent shear bands and large Vickers hardness (>1200 kg mm^?2). On the other hand, the Co-rich ribbons with greater ductility (x = 0.025, 0.05) exhibit a vein pattern filled with voids (features ～2–11 µm), extensive shear banding and lower Vickers hardness (<1050 kg mm^?2).     

A new type of periodie boundary condition useful for high-temperature atomistic simulations of grain boundaries: Applications in semiconductors

A new type of boundary condition, named Möbius or antiperiodic boundary conditions, is proposed and tested, both analytically and within the context of numerical simulations. It is shown that these boundary conditions are very useful for twist grain boundary atomistic simulations. By contrast to the use of the ordinary Born von Kármán periodic boundary conditions, they allow only one grain boundary per box instead of two. The risk of migration and overinteraction of two grain boundaries at high temperature is thus avoided while more complex grain boundaries can also be tackled at the same computer price. Such examples are presented and discussed.     

Dynamic characteristics of nanoindentation in Ni:A molecular dynamics simulation study

In this work,three-dimensional molecular dynamics simulation is carried out to elucidate the nanoindentation behaviour of single crystal Ni.The substrate indenter system is modelled using hybrid interatomic potentials including the manybody potential(embedded atom method) and two-body Morse potential.The spherical indenter is chosen,and the simulation is performed for different loading rates from 10 m/s to 200 m/s.Results show that the maximum indentation load and hardness of the system increase with the increase of velocity.The effect of indenter size on the nanoindentation response is also analysed.It is found that the maximum indentation load is higher for the large indenter whereas the hardness is higher for the smaller indenter.Dynamic nanoindentation is carried out to investigate the behaviour of Ni substrate to multiple loading-unloading cycles.It is observed from the results that the increase in the number of loading unloading cycles reduces the maximum load and hardness of the Ni substrate.This is attributed to the decrease in recovery force due to defects and dislocations produced after each indentation cycle.     

Development of interatomic potentials appropriate for simulation of liquid and glass properties of NiZr2 alloy

A new interatomic potential for the Ni–Zr system is presented. This potential was developed specifically to match experimental scattering data from Ni, Zr and NiZr₂ liquids. Both ab initio and published thermodynamic data were used to optimise the potential to study the liquid and amorphous structure of the NiZr₂ alloy. This potential has the C ₁₆ phase, being more stable than C _11b phase in the NiZr₂ alloy, consistent with experiments. The potential leads to the correct glass structure in the molecular dynamics simulation and, therefore, can be used to study the liquid–glass transformation in the NiZr₂ alloy.     

Interfaces in Iron-Rich Ordered Fe-Al Alloys: An Atomic-Scale Simulation Study

The aim of this paper is to investigate the low-temperature structure of interfaces (the (3 1 0) [0 0 1] symmetrical tilt grain boundary (GB) and the (3 1 0) surface) in stoichiometric ordered Fe-Al alloys with B2 and DO₃ structures. In both alloys, (i) the GBs cannot be realistically described by geometrical models, (ii) GBs and surfaces show strong segregation effects. A simple independent-defect model cannot be applied: the interactions between point defects sometimes lead to results opposite to those predicted from the formation energies of isolated point defects. The excess energies and configurations of the most stable interface variants are determined. All interfaces show a tendency to Al segregation except the B2 GB for which the most stable structure is an Fe-rich one. The interface structures are more complex in the DO₃ than in the B2 alloy, with a high multiplicity of DO₃ configurations with close energies. Finally, values of the GB and surface energies are introduced into a Griffith model of brittle fracture, in order to assess the trends of both alloys to intergranular fracture. Comparisons are also drawn with the similar Ni-Al ordered alloys.     

Magnetic Properties of Grain Boundaries of Nanocrystalline Ni and of Ni Precipitates in Nanocrystalline NiCu Alloys

Perturbed γγ-angular correlation spectroscopy (PAC) was used to investigate nanocrystalline Ni and NiCu alloys, which are prepared by pulsed electrodeposition (PED). Using diffusion for doping nanocrystalline Ni with ¹¹¹In four different ordered grain boundary structures are observed, which are characterized by unique electric field gradients. The incorporation of ¹¹¹In on substitutional bulk sites of Ni is caused by moving grain boundaries below 1000 K and by volume diffusion above 1000 K. The nanocrystalline NiCu alloys prepared by PED are microscopically inhomogeneous as observed by PAC. In contrast, this inhomogeneity cannot be detected by X-ray diffraction. The influence of the temperature of the electrolyte, the current density during deposition, and the optional addition of saccharin to the electrolyte on the homogeneity of nanocrystalline NiCu alloys was investigated. This revised version was published online in September 2006 with corrections to the Cover Date.     

高质量纳米ZnO薄膜的光致发光特性研究

报道了利用低压-金属有机物化学气相沉积技术生长纳米ZnS薄膜，然后，将ZnS薄膜在氧气中于800℃温度下进行热氧化制备高质量纳米ZnO薄膜.x射线衍射结果表明，纳米ZnO薄膜具有六角纤锌矿多晶结构.室温下观察到一束强的紫外(3.26 eV) 光致发光和很弱的深能级发射.根据激子峰的半高宽度与温度的关系确定了激子-纵向光学声子（LO）的耦合强度（ГLO）.由于量子限域效应使ГLO减少较多. 关键词： 光致发光 热氧化 激子 纳米ZnO薄膜     

Electrical activity and structural configuration of transition metals (Cu, Ni, Fe) in silicon bicrystals

Deep-level transient spectroscopy and electron-beam-induced current measurements have been performed on =25, =13 and =9 silicon bicrystals contaminated with Cu, Ni or Fe. Among the studied grain boundaries, only the heat-treated =25 showed a barrier effect which seems to originate, for the uncontaminated samples, from the structural defects induced by the rapid disorder with temperature. An extrinsic origin would start to prevail, however, upon formation of impurity precipitates in the contaminated specimens. The evolution of the barrier height (BH) and the electronic interface states with the annealing temperature, T _a, allowed one to infer that the rectifying behavior of the buried silicides could be compared to that of the same silicide layer grown on a silicon surface. This was strongly suggested by the systematic lowering of the BH with T _a which is consistent with the increasingly greater contribution of type-A NiSi₂ silicides with their low BH. The opposite behavior of the Cu and Fe cases appeared due to the tendency towards the pinning of the Fermi level near the mid-gap. For EBIC, the different aspects of the contrast were assigned to the various microstructures of the interfaces. Finally, the formation of impurity particles were found to involve a net degradation of the minority-carrier diffusion length, in addition to a concomitant and consistent increase of both recombination velocity and density of interface states.     

Free volumes in bulk nanocrystalline metals studied by the complementary techniques of positron annihilation and dilatometry

Free-volume type defects, such as vacancies, vacancy-agglomerates, dislocations, and grain boundaries represent a key parameter in the properties of ultrafine-grained and nanocrystalline materials. Such free-volume type defects are introduced in high excess concentration during the processes of structural refinement by severe plastic deformation. The direct method of time-differential dilatometry is applied in the present work to determine the total amount and the kinetics of free volume by measuring the irreversible length change upon annealing of bulk nanocrystalline metals (Fe, Cu, Ni) prepared by high-pressure torsion (HPT). In the case of HPT-deformed Ni and Cu, distinct substages of the length change upon linear heating occur due to the loss of grain boundaries in the wake of crystallite growth. The data on dilatometric length change can be directly related to the fast annealing of free-volume type defects studied by in situ Doppler broadening measurements performed at the high-intensity positron beam of the FRM II (Garching, Munich, Germany).     

纳米晶永磁中面缺陷对畴壁钉扎机理的研究

畴壁钉扎模型的矫顽力可以表示为Hc=αpink2K1/μ0Ms-NeffMs，计算了微结构参数αpink随面缺陷内磁性参数A′和K′1的变化情况.结果表明，αpink可以在很宽的范围里取值.结合纳米单相Nd2Fe14B磁体，研究了晶粒边界的磁性参数和晶界厚度对αpink的影响，当A′/A=05，K′1/K1=01，以及晶界厚度r0＝332nm时，αpink最大.同时研究了纳米复相Nd2Fe14B/α-Fe磁体的αpink随α-Fe晶粒尺寸r0的变化情况，当r0＝7nm时，αpink最大. 关键词： 面缺陷 矫顽力 钉扎机理 纳米晶     

Effect of grain boundary structures on the behavior of He defects in Ni: An atomistic study

We investigated the effect of grain boundary structures on the trapping strength of He_N(N is the number of helium atoms) defects in the grain boundaries of nickel. The results suggest that the binding energy of an interstitial helium atom to the grain boundary plane is the strongest among all sites around the plane. The He_N defect is much more stable in nickel bulk than in the grain boundary plane. Besides, the binding energy of an interstitial helium atom to a vacancy is stronger than that to a grain boundary plane. The binding strength between the grain boundary and the He _N defect increases with the defect size. Moreover, the binding strength of the He_N defect to the Σ3(12)[110] grain boundary becomes much weaker than that to other grain boundaries as the defect size increases.